The transmissions, in particular by satellite in the Ka (K-above) and EHF (Extremely High Frequency) bands are sensitive to various different phenomena that can degrade the budget of the link between a transmitter and a receiver. These phenomena can lead to very rapid variations, such as masking or interference. The connection of the link can then be reduced by several decibels per second.
Other phenomena, such as weather related variations, in particular rain fade or antenna pointing errors have rapid effects that lead to a reduction of the gain by a few tenths of decibels per second.
Finally, other phenomena, such as the geographic location or situation of the receiver when it is mobile may result in slower variations of the gain of the link of the order of a hundredth of a decibel per second.
In order to be better adapted to these variations, mechanisms for adapting the modes of modulation and coding have been implemented. The goal is to dynamically adapt the parameters of the waveform so as to be well adapted to the link budget. This mechanism is known by the acronym AMC in English, for “Adaptive Modulation and Coding”.
As it is known per se, the AMC mechanism makes it possible, by comparing the signal to noise ratio to the baseline reference values to define the mode of modulation and coding adapted to the conditions of the link.
The propagation of information between the entities of the chain of transmission for transmitting the information pertaining to the state of communication and orders of change in modulation and coding requires a substantial amount of time, so that when the signal to noise ratio decreases, it takes a certain amount of time for the transmitter to be able to react to this decrease.
In order to ensure that the signal to noise ratio of the link is never less than a baseline reference signal to noise ratio necessary for the receiver, it is a well known practice to provide for a margin, added to the baseline reference signal to noise ratio in order to anticipate the losses of the link budget and to be able to change the modulation and coding early enough before the conditions become far too degraded.
This margin is called AMC margin.
The AMC margin depends on the worst case scenario variation of link budget to which the transmission system must be resistant as well as the reaction time of the system.
In general, the AMC margin is static and is of the order of 2 to 3 decibels for transmissions in the Ka band and the AMC margin may be higher in the EHF band.
When the conditions for signal propagation are stable, typically with a clear sky, the margin is unnecessary since the signal to noise ratio does not vary. The transmission power is thus 2 to 3 decibels higher than necessary thereby causing a decrease of the speed or the bandwidth of the order of 50% to 100%.
It is a known technique to make the AMC margin vary based on the historical information related to the change in the signal to noise ratio.
These solutions have the drawback of sometimes impose unnecessarily high AMC margins. The variation in signal to noise ratio may be of the order of 20 decibels, leading to the possibility of retaining an AMC margin of around several decibels, without this improving the communication, the phenomena deemed to have caused the variation in signal to noise ratio having been very brief and thus not having needed to be compensated for by a change in modulation or coding.
The aim of the invention is to provide a method of transmission with adaptive modulation and coding in which the changing of the AMC margin:                makes possible the optimisation of the transmission power when conditions for signal propagation are stable        does not lead to changes in the mode of coding or modulation considered unnecessary, in particular in the event of masking or interference.        